Field of the Invention
The present invention relates to a load control device for a high-efficiency light source, such as a light-emitting diode (LED) light source or a compact fluorescent lamp, and more particularly, to a controllable-load circuit for use in an LED driver for controlling the intensity of an LED light source.
Description of the Related Art
Light-emitting diode (LED) light sources are often used in place of or as replacements for conventional incandescent, fluorescent, or halogen lamps, and the like. LED light sources comprise a plurality of light-emitting diodes mounted on a single structure and provided in a suitable housing. LED light sources are typically more efficient and provide longer operational lives as compared to incandescent, fluorescent, and halogen lamps. In order to illuminate properly, an LED driver control device (i.e., an LED driver) is typically coupled between an alternating-current (AC) source and the LED light source for regulating the power supplied to the LED light source. The LED driver may regulate either the voltage provided to the LED light source to a particular value, the current supplied to the LED light source to a specific peak current value, or may regulate both the current and voltage.
LED drivers may be controlled by standard two-wire dimmer switches. A typical dimmer switch comprises a bidirectional semiconductor switch, e.g., a thyristor (such as a triac) or two field-effect transistors (FETs) in anti-series connection. The bidirectional semiconductor switch is coupled in series between the AC power source and the LED driver and is controlled to be conductive and non-conductive for portions of a half-cycle of the AC power source to thus control the amount of power delivered to the LED driver (e.g., using a phase-control dimming technique). A “smart” dimmer switch (i.e., a digital dimmer switch) comprises a microprocessor (or similar controller) for controlling the semiconductor switch and a power supply for powering the microprocessor. In order to properly charge, the power supply of a two-wire dimmer switch must conduct a charging current ICHRG from the AC power source through the LED driver when the bidirectional semiconductor switch of the dimmer switch is non-conductive each half-cycle to provide an appropriate amount of voltage to the microprocessor (and any other low voltage circuitry).
Generally, dimmer switches use either a forward phase-control dimming technique or a reverse phase-control dimming technique in order to control when the bidirectional semiconductor switch is rendered conductive and non-conductive to thus control the power delivered to the LED driver. With forward phase-control dimming, the bidirectional semiconductor switch is turned on at some point within each AC line voltage half-cycle and remains on until the end of the half cycle. Forward phase-control dimming is often used to control the power delivered to a resistive or inductive load, which may include, for example, a magnetic low-voltage transformer or an incandescent lamp. Since a triac becomes non-conductive when the magnitude of the current conducted through the triac decreases to approximately zero amps, triacs are typically used for forward phase-control dimming. With reverse phase-control dimming, the bidirectional semiconductor switch is turned on at the zero-crossing of the AC line voltage and turned off at some point within each half-cycle of the AC line voltage. Reverse phase-control dimming is often used to control the power delivered to a capacitive load, which may include, for example, an electronic low-voltage transformer. Since the bidirectional semiconductor switch must be rendered conductive at the beginning of the half-cycle, and must be able to be turned off within the half-cycle, reverse phase-control dimming typically requires that the dimmer switch have two FETs in anti-serial connection, or the like.
As previously mentioned, the bidirectional semiconductor switch of the dimmer switch may be implemented as a thyristor, such as a triac or two silicon-controlled rectifiers (SCRs) coupled in anti-parallel connection. Thyristors are typically characterized by a rated latching current and a rated holding current. The current conducted through the main terminals of the thyristor must exceed the latching current for the thyristor to become fully conductive. In addition, the current conducted through the main terminals of the thyristor must remain above the holding current for the thyristor to remain in full conduction. However, when a dimmer switch is controlling a high-efficiency electrical load, such as an LED light source, the current conducted through the main terminals of the thyristor may likely not exceed the rated latching current or the rated holding current.
Some prior art lighting control systems for LED light engines have included additional load circuits coupled in parallel with the LED light engines for conducting enough current to ensure that the latching current and holding current of the thyristor in the dimmer switch are exceeded. In addition, the load circuit may also conduct the charging current of the power supply of the dimmer switch to ensure that the power supply is able to stay charged. The load circuit may comprise, for example, a fixed load, such as a resistor, or a variable load circuit that can provide two different load characteristics during each half-cycle. An example of a prior art variable load circuit is described in greater detail in co-pending, commonly-assigned U.S. patent application Ser. No. 12/438,587, filed Feb. 24, 2009, entitled VARIABLE LOAD CIRCUITS FOR USE WITH LIGHTING CONTROL DEVICES, the entire disclosure of which is hereby incorporated by reference. The prior art load circuits conduct current through the connected dimmer switches at all times when the load circuits are installed. However, it may not be necessary for the load circuits to conduct the current through the dimmer switch at all times. In other words, the prior art load circuits may unnecessarily waste power by conducting current through the dimmer switch when not required. Therefore, there is a need for a controllable-load circuit that is able to conduct the appropriate amount of current through the connected dimmer switch only when needed.